include ../_util-fns

:marked
  The Tour of Heroes is evolving and we anticipate adding more components in the near future.

  Multiple components will need access to hero data and we don't want to copy and
  paste the same code over and over.
  Instead, we'll create a single reusable data service and learn to
  inject it in the components that need it.

  Refactoring data access to a separate service keeps the component lean and focused on supporting the view.
  It also makes it easier to unit test the component with a mock service.

  Because data services are invariably asynchronous,
  we'll finish the chapter with a **!{_Promise}**-based version of the data service.

  Run the <live-example></live-example> for this part.

.l-main-section
:marked
  ## Where We Left Off
  Before we continue with our Tour of Heroes, let’s verify we have the following structure.
  If not, we’ll need to go back and follow the previous chapters.

.filetree
  .file angular_tour_of_heroes
  .children
    .file lib
    .children
      .file app_component.dart
      .file hero.dart
      .file hero_detail_component.dart
    .file web
    .children
      .file index.html
      .file main.dart
      .file styles.css
    .file pubspec.yaml
:marked
  ### Keep the app compiling and running
  Open a terminal/console window.
  Start the Dart compiler, watch for changes, and start our server by entering the command:

code-example(language="bash").
  pub serve

:marked
  The application runs and updates automatically as we continue to build the Tour of Heroes.

  ## Creating a Hero Service
  Our stakeholders have shared their larger vision for our app.
  They tell us they want to show the heroes in various ways on different pages.
  We already can select a hero from a list.
  Soon we'll add a dashboard with the top performing heroes and create a separate view for editing hero details.
  All three views need hero data.

  At the moment the `AppComponent` defines mock heroes for display.
  We have at least two objections.
  First, defining heroes is not the component's job.
  Second, we can't easily share that list of heroes with other components and views.

  We can refactor this hero data acquisition business to a single service that provides heroes, and
  share that service with all components that need heroes.

  ### Create the HeroService
  Create a file in the `lib` folder called `hero_service.dart`.
.l-sub-section
  :marked
    We've adopted a convention in which we spell the name of a service in lowercase followed by `_service`.
    If the service name were multi-word, we'd spell the base filename in lower underscore case (also called [snake_case](../guide/glossary.html#snake_case)).
    The `SpecialSuperHeroService` would be defined in the `special_super_hero_service.dart` file.
:marked
  We name the class `HeroService`.

+makeExample('toh-4/dart/lib/hero_service_1.dart', 'empty-class', 'lib/hero_service.dart (starting point)')(format=".")

:marked
  ### Injectable Services
  Notice that we used an `@Injectable()` annotation.
.callout.is-helpful
  :marked
    **Don't forget the parentheses!** Neglecting them leads to an error that's difficult to diagnose.
:marked
  Dart sees the `@Injectable()` annotation and emits metadata about our service,
  metadata that Angular may need to inject other dependencies into this service.

  The `HeroService` doesn't have any dependencies *at the moment*. Add the annotation anyway.
  It is a "best practice" to apply the `@Injectable()` annotation ​*from the start*​
  both for consistency and for future-proofing.

:marked
  ### Getting Heroes
  Add a `getHeroes` method stub.
+makeExample('toh-4/dart/lib/hero_service_1.dart', 'getHeroes-stub', 'lib/hero_service.dart (getHeroes stub)')(format=".")
:marked
  We're holding back on the implementation for a moment to make an important point.

  The consumer of our service doesn't know how the service gets the data.
  Our `HeroService` could get `Hero` data from anywhere.
  It could get the data from a web service or local storage
  or from a mock data source.

  That's the beauty of removing data access from the component.
  We can change our minds about the implementation as often as we like,
  for whatever reason, without touching any of the components that need heroes.


  ### Mock Heroes
  We already have mock `Hero` data sitting in the `AppComponent`.  It doesn't belong there. It doesn't belong *here* either.
  We'll move the mock data to its own file.

  Cut the `mockHeroes` list from `app_component.dart` and paste it to a new file in the `lib` folder named `mock_heroes.dart`.
  We copy the `import 'hero.dart'` statement as well because the heroes list uses the `Hero` class.

+makeExample('toh-4/dart/lib/mock_heroes.dart', null, 'lib/mock_heroes.dart')
:marked
  Meanwhile, back in `app_component.dart` where we cut away the `mockHeroes` list,
  we leave behind an uninitialized `heroes` property:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'heroes-prop', 'lib/app_component.dart (heroes property)')(format=".")
:marked
  ### Return Mocked Heroes
  Back in the `HeroService`  we import the mock `mockHeroes` and return it from the `getHeroes` method.
  Our `HeroService` looks like this:
+makeExample('toh-4/dart/lib/hero_service_1.dart', 'final', 'lib/hero_service.dart')(format=".")
:marked
  ### Use the Hero Service
  We're ready to use the `HeroService` in other components starting with our `AppComponent`.

  We begin, as usual, by importing the thing we want to use, the `HeroService`.
+makeExcerpt('toh-4/dart/lib/app_component.dart', 'hero-service-import')
:marked
  Importing the service allows us to *reference* it in our code.
  How should the `AppComponent` acquire a runtime concrete `HeroService` instance?

  ### Do we *new* the *HeroService*? No way!
  We could create a new instance  of the `HeroService` with `new` like this:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'new-service')(format=".")
:marked
  That's a bad idea for several reasons including

  * Our component has to know how to create a `HeroService`.
  If we ever change the `HeroService` constructor,
  we'll have to find every place we create the service and fix it.
  Running around patching code is error prone and adds to the test burden.

  * We create a new service each time we use `new`.
  What if the service should cache heroes and share that cache with others?
  We couldn't do that.

  * We're locking the `AppComponent` into a specific implementation of the `HeroService`.
  It will be hard to switch implementations for different scenarios.
  Can we operate offline?
  Will we need different mocked versions under test?
  Not easy.

  *What if ... what if ... Hey, we've got work to do!*

  We get it. Really we do.
  But it is so ridiculously easy to avoid these problems that there is no excuse for doing it wrong.

  ### Inject the *HeroService*

  Three lines replace the one line of *new*:
  1. We add a property.
  1. We add a constructor that sets the property.
  1. We add to the component's `providers` metadata.

  Here are the property and the constructor:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'ctor', 'lib/app_component.dart (constructor)')(format='.')
:marked
  The constructor does nothing except set the `_heroService`
  property. The `HeroService` type of `_heroService`
  identifies the constructor's parameter as
  a `HeroService` injection site.

  Now Angular will know to supply an instance of the `HeroService` when it creates a new `AppComponent`.

  Angular has to get that instance from somewhere. That's the role of the Angular *Dependency Injector*.
  The **Injector** has a **container** of previously created services.
  Either it finds and returns a pre-existing `HeroService` from its container or it creates a new instance, adds
  it to the container, and returns it to Angular.

.l-sub-section
  :marked
    Learn more about Dependency Injection in the [Dependency Injection](../guide/dependency-injection.html) chapter.
:marked
  The *injector* does not know yet how to create a `HeroService`.
  If we ran our code now, Angular would fail with an error:
code-example(format="nocode").
    EXCEPTION: No provider for HeroService! (AppComponent -> HeroService)
:marked
  We have to teach the *injector* how to make a `HeroService` by registering a `HeroService` **provider**.
  Do that by adding the following `providers` parameter to the bottom of the component metadata
  in the `@Component` annotation.

+makeExcerpt('toh-4/dart/lib/app_component_1.dart', 'providers')
:marked
  The `providers` parameter  tells Angular to create a fresh instance of the `HeroService` when it creates a new `AppComponent`.
  The `AppComponent` can use that service to get heroes and so can every child component of its component tree.
<a id="child-component"></a>
.l-sub-section
  :marked
    ### Services and the component tree

    Recall that the `AppComponent` creates an instance of `HeroDetail` by virtue of the
    `<my-hero-detail>` tag at the bottom of its template. That `HeroDetail` is a child of the `AppComponent`.

    If the `HeroDetailComponent` needed its parent component's `HeroService`,
    it would ask Angular to inject the service into its constructor which would look just like the one for `AppComponent`:
  +makeExample('toh-4/dart/lib/app_component_1.dart', 'ctor', 'lib/hero_detail_component.dart (constructor)')(format=".")
  :marked
    The `HeroDetailComponent` must *not* repeat its parent's `providers` list! Guess [why](#shadow-provider).

    The `AppComponent` is the top level component of our application.
    There should be only one instance of that component and only one instance of the `HeroService` in our entire app.
:marked
  ### *getHeroes* in the *AppComponent*
  We've got the service in a `_heroService` private variable. Let's use it.

  We pause to think. We can call the service and get the data in one line.
+makeExample('toh-4/dart/lib/app_component_1.dart', 'get-heroes')(format=".")
:marked
  We don't really need a dedicated method to wrap one line.  We write it anyway:
+makeExcerpt('toh-4/dart/lib/app_component_1.dart', 'getHeroes')

<a id="oninit"></a>
:marked
  ### The *ngOnInit* Lifecycle Hook
  `AppComponent` should fetch and display heroes without a fuss.
  Where do we call the `getHeroes` method? In a constructor? We do *not*!

  Years of experience and bitter tears have taught us to keep complex logic out of the constructor,
  especially anything that might call a server as a data access method is sure to do.

  The constructor is for simple initializations like wiring constructor parameters to properties.
  It's not for heavy lifting. We should be able to create a component in a test and not worry that it
  might do real work &mdash; like calling a server!  &mdash; before we tell it to do so.

  If not the constructor, something has to call `getHeroes`.

  Angular will call it if we implement the Angular **ngOnInit** *Lifecycle Hook*.
  Angular offers a number of interfaces for tapping into critical moments in the component lifecycle:
  at creation, after each change, and at its eventual destruction.

  Each interface has a single method. When the component implements that method, Angular calls it at the appropriate time.
.l-sub-section
  :marked
    Learn more about lifecycle hooks in the [Lifecycle Hooks](../guide/lifecycle-hooks.html) chapter.
:marked
  Here's the essential outline for the `OnInit` interface:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'on-init', 'lib/app_component.dart (ngOnInit stub)')(format=".")
:marked
  We write an `ngOnInit` method with our initialization logic inside and leave it to Angular to call it
  at the right time. In our case, we initialize by calling `getHeroes`.
+makeExcerpt('toh-4/dart/lib/app_component_1.dart', 'ng-on-init')
:marked
  Our application should be running as expected, showing a list of heroes and a hero detail view
  when we click on a hero name.

  We're getting closer. But something isn't quite right.

  <a id="async"></a>
  ## Async Services and !{_Promise}
  Our `HeroService` returns a list of mock heroes immediately.
  Its `getHeroes` signature is synchronous
+makeExample('toh-4/dart/lib/app_component_1.dart', 'get-heroes')(format=".")
:marked
  Ask for heroes and they are there in the returned result.

  Someday we're going to get heroes from a remote server. We don’t call http yet, but we aspire to in later chapters.

  When we do, we'll have to wait for the server to respond and we won't be able to block the UI while we wait,
  even if we want to (which we shouldn't) because the browser won't block.

  We'll have to use some kind of asynchronous technique and that will change the signature of our `getHeroes` method.

  We'll use *!{_Promise}s*.

  ### The Hero Service returns a !{_Promise}

  We ask an asynchronous service to do some work and give us the result in the !{_Promise}.
  The service does that work (somewhere) and eventually it updates the !{_Promise} with the results of the work or an error.
.l-sub-section
  :marked
    We are simplifying. Learn about !{_Promise}s in the tutorial
    [Asynchronous Programming: Futures](https://www.dartlang.org/docs/tutorials/futures/).
:marked
  Update the `HeroService` with this !{_Promise}-returning `getHeroes` method:
+makeExample('toh-4/dart/lib/hero_service.dart', 'get-heroes', 'lib/hero_service.dart (excerpt)')(format=".")
:marked
  We're still mocking the data. We're simulating the behavior of an ultra-fast, zero-latency server,
  by returning a !{_Promise} that will quickly resolve with our mock heroes as the result.

.l-sub-section
  :marked
    Marking the method's body with `async` makes the method immediately return a `Future` object.
    That !{_Promise} later completes with the method's return value.
    For more information on async functions, see
    [Declaring async functions](https://www.dartlang.org/docs/dart-up-and-running/ch02.html#async) in the Dart language tour.

:marked
  ### Act on the !{_Promise}
  Returning to the `AppComponent` and its `getHeroes` method, we see that it still looks like this:
+makeExample('toh-4/dart/lib/app_component_1.dart', 'getHeroes', 'lib/app_component.dart (getHeroes - old)')(format=".")
:marked
  As a result of our change to `HeroService`, we're now setting `heroes` to a !{_Promise} rather than a list of heroes.

  We have to change our implementation to *act on the !{_Promise} when it resolves*.
  We can *await* for the !{_Promise} to resolve, and then display the heroes:
+makeExample('toh-4/dart/lib/app_component.dart', 'get-heroes', 'lib/app_component.dart (getHeroes - revised)')(format=".")
:marked
  Our code waits until the !{_Promise} completes, and then
  sets the component's `heroes` property to the list of heroes returned by the service. That's all there is to it!

  Our app should still be running, still showing a list of heroes, and still
  responding to a name selection with a detail view.
.l-sub-section
  :marked
    Check out the "[Take it slow](#slow)" appendix to see what the app might be like with a poor connection.
:marked
  ### Review the App Structure
  Let’s verify that we have the following structure after all of our good refactoring in this chapter:

.filetree
  .file angular_tour_of_heroes
  .children
    .file lib
    .children
      .file app_component.dart
      .file hero.dart
      .file hero_detail_component.dart
      .file hero_service.dart
      .file mock_heroes.dart
    .file web
    .children
      .file index.html
      .file main.dart
      .file styles.css
    .file pubspec.yaml
:marked
  Here are the code files we discussed in this chapter.

+makeTabs(`
  toh-4/dart/lib/hero_service.dart,
  toh-4/dart/lib/app_component.dart,
  toh-4/dart/lib/mock_heroes.dart
  `,'',`
  lib/hero_service.dart,
  lib/app_component.dart,
  lib/mock_heroes.dart
  `)
:marked
  ## The Road We’ve Travelled
  Let’s take stock of what we’ve built.

  * We created a service class that can be shared by many components.
  * We used the `ngOnInit` Lifecycle Hook to get our heroes when our `AppComponent` activates.
  * We defined our `HeroService` as a provider for our `AppComponent`.
  * We created mock hero data and imported them into our service.
  * We designed our service to return a !{_Promise} and our component to get our data from the !{_Promise}.


  ### The Road Ahead
  Our Tour of Heroes has become more reusable using shared components and services.
  We want to create a dashboard, add menu links that route between the views, and format data in a template.
  As our app evolves, we’ll learn how to design it to make it easier to grow and maintain.

  We learn about Angular Component Router and navigation among the views in the [next tutorial](toh-pt5.html) chapter.

.l-main-section
<a id="slow"></a>
:marked
  ### Appendix: Take it slow

  We can simulate a slow connection.

  Add the following `getHeroesSlowly` method to the `HeroService`:
+makeExample('toh-4/dart/lib/hero_service.dart', 'get-heroes-slowly', 'lib/hero_service.dart (getHeroesSlowly)')(format=".")
:marked
  Like `getHeroes`, it also returns a !{_Promise}.
  But this !{_Promise} waits 2 seconds before resolving the !{_Promise} with mock heroes.

  Back in the `AppComponent`, replace
  `_heroService.getHeroes` with `_heroService.getHeroesSlowly`
  and see how the app behaves.

.l-main-section
<a id="shadow-provider"></a>
:marked
  ### Appendix: Shadowing the parent's service

  We stated [earlier](#child-component) that if we injected the parent `AppComponent` `HeroService`
  into the `HeroDetailComponent`, *we must not add a providers list* to the `HeroDetailComponent` metadata.

  Why?  Because that tells Angular to create a new instance of the `HeroService` at the `HeroDetailComponent` level.
  The `HeroDetailComponent` doesn't want its *own*  service instance; it wants its *parent's* service instance.
  Adding the `providers` list creates a new service instance that shadows the parent instance.

  Think carefully about where and when to register a provider.
  Understand the scope of that registration. Be careful not to create a new service instance at the wrong level.
